M0: backend skeleton — Quart factory, async DB, auth, ACL spine, migrations

Foundation & Identity backend for ThoughtSync:
- Quart app factory (create_app) with /api/health + SPA history-fallback
- async SQLAlchemy 2.0 + asyncpg engine/session (lazy; boots without a DB)
- native email+password auth via signed-cookie session (register/login/logout/me
  + login_required guard); bcrypt password hashing (72-byte safe)
- multi-user sharing-ACL spine (rule 47): users, groups, group_members, and a
  polymorphic shares table + visible_to_user() SQL predicate (owner OR direct
  share OR group share) that M1's notes will scope through
- Alembic async env (adapted from family pattern) + 0001 foundation migration
- DB-free unit tests (app/health/auth-guard, password roundtrip, ACL compile)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

tests/conftest.py

@@ -0,0 +1,13 @@
+import pytest
+
+from thoughtsync.config import Config
+
+
+@pytest.fixture(autouse=True)
+def _isolated_data_dir(tmp_path, monkeypatch):
+    """Point DATA_DIR at a writable temp dir so create_app() can generate its
+    signing key without needing /var/thoughtsync to exist (DB-free unit tests)."""
+    data_dir = tmp_path / "data"
+    monkeypatch.setattr(Config, "DATA_DIR", str(data_dir))
+    monkeypatch.setattr(Config, "MEDIA_ROOT", str(data_dir / "media"))
+    yield